Priestley-Taylor Equation Research Articles

Estimating and validating basin-scale actual evapotranspiration using MODIS images and hydrologic models

An algorithm for estimating daily spatial actual evapotranspiration (ET) from remotely sensed MODIS data is presented. It is based on the surface energy balance scheme and the modified Priestley–Taylor equation, and has been applied to the MODIS data acquired during growing seasons over the Laohahe River basin, northeastern China. Spatial distributed mapping of daily ET for 22 clear sky days in the year of 2000 from MODIS images over the study area were obtained. In order to validate ET values estimated from MODIS data, regional daily ET values were calculated using the lumped modified Xinanjiang hydrologic model and distributed SWAT model based on the water balance scheme, respectively. The relationship between actual daily ET estimated from MODIS images and basin-scale ET calculated from the hydrologic model were in good agreement with acceptable correlation coefficient. The results suggested that the algorithm is applicable and operational for estimating and mapping basin-scale distributed daily actual ET over the study area. In order to use the algorithm proposed by this paper for water resource management and agricultural decision making, the algorithm should be validated using more data and be tested under different environment and different land use scenario conditions in future work.

Estimation of evapotranspiration for ungauged areas using MODIS measurements and GLDAS data

Abstract On base of Priestley-Taylor Equation combined with the Ts/NDVI space, namely, surface temperature-Normalized Difference Vegetation Index feature space, MODIS (Moderate Resolution Imaging Spectroradiometer) products and GLDAS(Global Land Data Assimilation System) meteorological data are introduced to calculate evapotranspiration (ET) in ungauged basins. The whole procedure is independent of field data. It was then applied to Tekesi River Basin to estimate evapotranspiration from 2006 to 2009.Validation has been made by comparing the estimated ET with the Bowen Ratio measurements from 15th, August to 15th, September in 2009 at Zhaosu site, which shows good agreement between the estimated ET and the observed.

Discussion of “Case Study on the Accuracy and Cost/Effectiveness in Simulating Reference Evapotranspiration in West-Central Florida” by Michael Grant Exner-Kittridge and Mark Cable Rains

John D. Valiantzas Professor, Div. of Water Resource Management, Dept. of Natural Resources and Agricultural Engineering, Agricultural Univ. of Athens, 75 Iera Odos, 11855, Athens, Greece. E-mail: lhyd2vay@aua.gr The authors evaluated various alternative reference ET equations based on various combinations of measured meteorological data for their accuracy in conjunction with their corresponding cost. The evaluated equations concern various reduced-data versions of the ASCE-PM standardized procedures (Allen et al. 2005), versions of the Priestley-Taylor equation, the Abtew (1996) radiation-type empirical formula, and the Hargreaves temperature-based equation. The various combinations of measured meteorological data examined are (Rs, G, U, RH, T); (Rs, G, T); (Rs, U, RH, T); (Rs, T); (T) (Rs, G, T); and (Rs). However, the authors did not examine the case where the only measured data are (Rs, RH, T). This combination of measured data approaches economically more closely the (Rs, T) and (Rs) combinations than the (Rs, U, RH, T) combination

The JGrass-NewAge system for forecasting and managing the hydrological budgets at the basin scale: models of flow generation and propagation/routing

Abstract. This paper presents a discussion of the predictive capacity of the implementation of the semi-distributed hydrological modeling system JGrass-NewAge. This model focuses on the hydrological budgets of medium scale to large scale basins as the product of the processes at the hillslope scale with the interplay of the river network. The part of the modeling system presented here deals with the: (i) estimation of the space-time structure of precipitation, (ii) estimation of runoff production; (iii) aggregation and propagation of flows in channel; (v) estimation of evapotranspiration; (vi) automatic calibration of the discharge with the method of particle swarming. The system is based on a hillslope-link geometrical partition of the landscape, combining raster and vectorial treatment of hillslope data with vector based tracking of flow in channels. Measured precipitation are spatially interpolated with the use of kriging. Runoff production at each channel link is estimated through a peculiar application of the Hymod model. Routing in channels uses an integrated flow equation and produces discharges at any link end, for any link in the river network. Evapotranspiration is estimated with an implementation of the Priestley-Taylor equation. The model system assembly is calibrated using the particle swarming algorithm. A two year simulation of hourly discharge of the Little Washita (OK, USA) basin is presented and discussed with the support of some classical indices of goodness of fit, and analysis of the residuals. A novelty with respect to traditional hydrological modeling is that each of the elements above, including the preprocessing and the analysis tools, is implemented as a software component, built upon Object Modelling System v3 and jgrasstools prescriptions, that can be cleanly switched in and out at run-time, rather than at compiling time. The possibility of creating different modeling products by the connection of modules with or without the calibration tool, as for instance the case of the present modeling chain, reduces redundancy in programming, promotes collaborative work, enhances the productivity of researchers, and facilitates the search for the optimal modeling solution.

Seasonal energy and water balance of a Phragmites australis-dominated wetland in the Republican River basin of south-central Nebraska (USA)

Summary Climate and vegetation strongly influence the water cycle on local to regional scales. A change in the surface energy and water balance, especially in dry climatic regions, can have a significant impact on local water availability and, therefore, water resource management. The purpose of this study is to quantify the energy and water balance of a riparian wetland in a subhumid region of the central US, as well as the role of seasonal climate variability and vegetation phenology. The site is located in the Republican River basin in south-central Nebraska, where decreases in streamflow have been observed in recent decades. In an effort to reduce consumptive water use from evapotranspiration (ET), and thereby reclaim surface water, invasive species such as Phragmites australis have been removed throughout the riparian corridor of the river basin. In this study, we used energy/water balance monitoring stations, a Large Aperture Scintillometer (LAS), and numerous water and soil temperature probes to determine the energy and water balance during the 2009 growing season (April 11−October 3). Sensible heat flux was measured using the LAS, while ET was calculated as a residual of the energy balance (i.e., net radiation minus sensible heat flux and heat storage rates in the canopy, water, and soil). Rigorous quality control and uncertainty analyses were performed, and comparisons were also made with ET rates calculated via the simpler Priestley–Taylor method. Results of the energy budget analysis indicate that the average ET rate for the wetland during the growing season was 4.4 mm day −1 , with a maximum daily rate of 8.2 mm day −1 (occurring on June 29). Precipitation during the same 176-day period averaged 2.7 mm day −1 . Net radiation and vegetation phenology were found to be the two largest drivers of seasonal variability in ET. Sensible heat flux was significantly larger than latent heat flux early in the season, when standing vegetation in the wetland was still dry and brown. By late May and early June, however, Bowen ratios had declined well below 0.5 in response to greener and more abundant vegetation, higher transpiration rates, and reduced sensible heat flux. Heat storage rates in the wetland were dominated by changes in water temperature (as compared to soil or canopy heat storage) and comprised a significant portion of the hourly energy balance. On daily mean timescales, changes in the rate of heat storage corresponded to ∼13% of the variability in net radiation, while for the season-long average, the heat storage term was found to be essentially negligible. The Priestley–Taylor equation provided a reasonable estimate of ET during the height of the growing season but significantly overestimated ET during the beginning of the season (since it could not account for large sensible heat fluxes from the dry vegetation). Analysis of the wetland water balance showed seasonal variations in water level that were similar to changes in cumulative water inputs (i.e., precipitation minus ET). Portions of the season when the two curves had differing rates of change indicated periods of net water influx or outflux from other sources (primarily groundwater).

Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes

Reliable estimation of sensible heat flux (H) is important in energy balance models for quantifying evapotranspiration (ET). This study was conducted to evaluate the value of adding the Priestley-Taylor (PT) equation to the METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) model. METRIC was used to estimate energy fluxes for 10 Landsat images from the 2005, 2006 and 2007 crop growing seasons in south-central Nebraska, USA, where each image owing to recent rainfall exhibited high residual moisture content even at the hot pixel. The METRIC model performed satisfactorily for net radiation (Rn ) and soil heat flux (G) estimation with a root mean square error (RMSE) of 52 and 24 W m-2, respectively. A RMSE of 122 W m-2 for H indicated the limitation of the METRIC model in estimating H for high residual moisture content of the hot pixel (Alfalfa reference ET fraction, ET r F > 0.15). The modified METRIC model (wet METRIC or wMETRIC) incorporating the PT equation was applied to calculate H at the anchor pixels (hot and cold) for high residual moisture content of the hot pixel. The α coefficient of the PT equation was locally calibrated using hourly meteorological data from an automatic weather station and Rn and G data from a Bowen ratio flux tower. The mean α coefficient value was 1.14. The wMETRIC model reduced the RMSE of H from 122 to 64 W m-2 and that of latent heat flux, LE, from 163 to 106 W m-2. The RMSE of daily ET decreased from 1.7 to 1.1 mm d-1 with wMETRIC. The results indicate that treatment of anchor pixels for high residual moisture content with the PT approach gives improved estimation of H, LE and daily ET. It is recommended that the wMETRIC model be used for estimating ET if the hot pixel has high residual moisture (i.e. reference ET fraction > 0.15). Citation Singh, R. K. & Irmak, A. (2011) Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes. Hydrol. Sci. J. 56(5), 895–906.

Local Calibration of the Hargreaves and Priestley-Taylor Equations for Estimating Reference Evapotranspiration in Arid and Cold Climates of Iran Based on the Penman-Monteith Model

The Food and Agricultural Organization of the United Nations (FAO)-56 version of Penman-Monteith (PMF-56) model has been established as a standard for calculating reference evapotranspiration (ETo). An important constraint of application of the PMF-56 model is the requirement of solar radiation, wind speed, air temperature, and humidity data, which may not be available for a given location, especially in developing countries. The Hargreaves (HG) and Priestley-Taylor (P-T) equations are simple equations that require few weather data inputs, although regional calibration of the equations is needed for acceptable performance before applying them for ETo estimation. In this study, the HG and P-T equations were calibrated on the basis of the PMF-56 method in arid and cold climates of Iran using data from 12 stations during 1994–2005. After calibration of the HG equation, the average value of the adjusted HG coefficient for arid climate was 0.0031, which is about 34% higher than the original value (0.0023). Sim...

Modeling Potential Evapotranspiration of Two Forested Watersheds in the Southern Appalachians

Global climate change has direct impacts on watershed hydrology through altering evapotranspiration (ET) processes at multiple scales. There are many methods to estimate forest ET with models, but the most practical and the most popular one is the potential ET (PET) based method. However, the choice of PET methods for AET estimation remains challenging. This study explored ways to identify appropriate PET models for two small forested watersheds, one dominated by conifer plantation and one dominated by native naturally regenerated deciduous hardwoods, by using long-term hydrometeorological data collected at the Coweeta Hydrologic Laboratory in the humid Appalachians in the southeastern U.S. Our specific objectives were to: (1) contrast three common PET models (FAO-56 grass reference ET, Hamon PET, and Priestley-Taylor PET) and compare these PET estimates with measured AET at monthly and annual temporal scales, and (2)�derive correction factors for the FAO-56 grass reference ET and Hamon PET models at the monthly scale using the Priestley-Taylor equation as the standard method for estimating forest PET. We found that different PET models gave significantly different PET estimates. The Priestley-Taylor equation gave the most reasonable estimates of forest PET for both watersheds. We conclude that the uncorrected Hamon and FAO PET methods would cause large underestimates of forest PET. Annual PET rates of the conifer watershed were higher than those of the native deciduous watershed due to the lower albedo (thus higher net radiation) in the former compared to the latter. Monthly correction factors provided useful tools for forest PET estimation in those areas lacking climatic data (i.e., radiation, humidity, and wind speed).

Assessing temperature‐based PET equations under a changing climate in temperate, deciduous forests

AbstractDespite the availability of numerous approaches to estimate potential evapotranspiration (PET), temperature‐based PET equations such as the Hamon equation and Thornthwaite equation are still used to predict changes in hydrology in a changing climate as temperature is one of the primary reported outputs from general circulation models. To isolate the actual dependence of PET on temperature, we analysed meteorological and energy balance measurements from five AmeriFlux deciduous forest sites in the eastern United States during periods with minimal soil moisture control on transpiration. For all five sites, when PET measurements with similar net radiation are grouped, temperature does not correlate to PET within each group. Conversely when PET measurements with similar temperature are grouped, net radiation strongly correlates to PET within each group. In terms of assessing standard PET models, when dormant and growing season PET are separated, we found that the Priestley–Taylor equation (a model primarily dependent on net radiation) consistently explained more of the variation in PET than temperature‐based methods such as the Hamon equation (median R2 of 0·88 vs 0·66). We illustrate that the moderate ability of temperature‐based equations to predict PET arises from the correlation between temperature and net radiation when the meteorological observations are averaged over at least several days. However, we suggest that because temperature is not the fundamental driver of PET and because the relationship between temperature and net radiation underlying temperature‐based equations will shift with climate change, temperature‐based equations in their current state will likely exaggerate PET in a changing climate. Copyright © 2010 John Wiley & Sons, Ltd.

A Comparison of Methods for Estimating Open-Water Evaporation in Small Wetlands

We compared evaporation measurements from a floating pan, land pan, chamber, and the Priestley-Taylor (PT) equation. Floating pan, land pan, and meteorological data were collected from June 6 to July 21, 2005, at a small wetland in the Canadian River alluvium in central Oklahoma, USA. Evaporation measured with the floating pan compared favorably to 12 h chamber measurements. Differences between chamber and floating pan rates ranged from −0.2 to 0.3 mm, mean of 0.1 mm. The difference between chamber and land pan rates ranged from 0.8 to 2.0 mm, mean of 1.5 mm. The mean chamber-to-floating pan ratio was 0.97 and the mean chamber-to-land pan ratio was 0.73. The chamber-to-floating pan ratio of 0.97 indicates the use of a floating pan to measure evaporation in small limited-fetch water bodies is an appropriate and accurate method for the site investigated. One-sided Paired t-Tests indicate daily floating pan rates were significantly less than land pan and PT rates. A two-sided Paired t-Test indicated there was no significant difference between land pan and PT values. The PT equation tends to overestimate evaporation during times when the air is of low drying power and tends to underestimate as drying power increases.

Estimating spatially distributed monthly evapotranspiration rates by linear transformations of MODIS daytime land surface temperature data

Abstract. Under simplifying conditions catchment-scale vapor pressure at the drying land surface can be calculated as a function of its watershed-representative temperature (<Ts>) by the wet-surface equation (WSE, similar to the wet-bulb equation in meteorology for calculating the dry-bulb thermometer vapor pressure) of the Complementary Relationship of evaporation. The corresponding watershed ET rate, <ET>, is obtained from the Bowen ratio with the help of air temperature, humidity and percent possible sunshine data. The resulting (<Ts>,<ET>) pair together with the wet-environment surface temperature (<Tws>) and ET rate (ETw), obtained by the Priestley-Taylor equation, define a linear transformation on a monthly basis by which spatially distributed ET rates can be estimated as a sole function of MODIS daytime land surface temperature, Ts, values within the watershed. The linear transformation preserves the mean which is highly desirable. <Tws>, in the lack of significant open water surfaces within the study watershed (Elkhorn, Nebraska), was obtained as the mean of the smallest MODIS Ts values each month. The resulting period-averaged (2000–2007) catchment-scale ET rate of 624 mm/yr is very close to the water-balance derived ET rate of about 617 mm/yr. The latter is a somewhat uncertain value due to the effects of (a) observed groundwater depletion of about 1m over the study period caused by extensive irrigation, and; (b) the uncertain rate of net regional groundwater supply toward the watershed. The spatially distributed ET rates correspond well with soil/aquifer properties and the resulting land use type (i.e. rangeland versus center-pivot irrigated crops).

Predicting Incoming Solar Radiation and Its Application to Radiation-Based Equation for Estimating Reference Evapotranspiration

This paper presents an inverse square weighted interpolation for predicting the incoming solar radiation ( Rs ) from nearby weather stations. The predicted Rs is applied to the well-known Priestley-Taylor equation for estimating reference evapotranspiration ( ETo ) . This cross-validation estimated bias and error in the final model predictions of the Rs and ETo at the 21 meteorological weather stations in Korea Peninsula. The coefficient of determination and the root-mean-square error (RMSE) for monthly estimates of Rs was in the range of 0.83–0.95 and 17.90–76.34 MJ m−2 day−1 , respectively. The RMSE for monthly estimate of ETo values at inland and coastal areas was 11.08 and 15.01 mm respectively. The estimates of ETo using thus predicted Rs to provide reasonable accuracy. The study can provide further useful guidelines for crop production, water resources conservation, irrigation scheduling, and environmental assessment.

Effect of actual vapor pressure on estimating evapotranspiration at Serbia

Actual vapor pressure (VP) is an important parameter that is used in many evapotranspiration equations. However, vapor pressure is difficult to measure accurately. In the humid climate, the actual vapor pressure can be derived from minimum air temperature. The objectives of this study were: first, to estimate errors that can arise if VP data are not available and have to be estimated; second, to compare the Priestley-Taylor ET0 values computed under various levels of VP data availability; and third, to evaluate the reliability of Priestley-Taylor equation as compared to the FAO-56 Penman-Monteith method. The following main conclusions can be drawn: Estimated VP values generally were in closest agreement with measured VP values. The measurements of air humidity at humid locations are not indispensable for estimating reference evapotranspiration. The Priestley-Taylor method (with measured or estimated VP) provides the very good agreement with the evapotranspiration obtained by the FAO-56 Penman-Monteith method except windless locations.

Variability of complementary relationship and its mechanism on different time scales

In the complementary relationship (CR) between actual and potential evapotranspiration, wet environment evapotranspiration (Ew) is usually calculated using the Priestley-Taylor (P-T) equation. Based on the data obtained from 38 catchments of the Haihe River basin and the Weishan experiment site in Shandong Province of China, this study has found variations in the Priestley-Taylor parameter α (e.g. increase in values with increase in the amount of precipitation on annual scale, seasonal fluctuation noted to be at a maximum during winter and at a minimum during summer, and increased values in the forenoon and decreased values in the afternoon on an hourly scale). This paper explains that the inter-annual variation in α is due to the negative feedback of atmosphere in response to changes in actual increase in evapotranspiration, which is weakened because the atmosphere system is open. The parameter α undergoes a seasonal fluctuation brought about by seasonal changes in advection between continent and ocean; α likewise undergoes a daily variation as a result of atmospheric hysteresis in response to the changes in land surface energy.

Evaluation of Three Practical Methods for Estimating Daily Solar Radiation in Dry Climates

Three practical methods for estimating daily solar radiation in dry climates with air temperature and precipitation data as input were evaluated. The three equations only partially explained second order statistics such as variance and different correlations, so the interdependence with primary weather variables such as the daily maximum and minimum temperatures was not fully captured. The equations, however, may be useful for calculations that require solar radiation as input, such as the daily reference evapotranspiration according to Priestley-Taylor equation using the FAO guidelines expert consultation.

Evaluation of a MODIS-Based Potential Evapotranspiration Product at the Point Scale

Abstract This paper outlines the development of a continuous, daily time series of potential evapotranspiration (PET) using Moderate Resolution Imaging Spectroradiometer (MODIS) sensor data from the Terra satellite platform. The approach is based on the Priestley–Taylor equation, incorporating a daily net radiation model during cloudless days. A simple algorithm using “theoretical clear-sky” net radiation (incorporating daily cloud fraction and cloud optical thickness) and PET is then used to estimate net radiation and PET under cloudy conditions. The method requires minimal ground-based observations for initial calibration of regional radiation algorithm coefficients. Point-scale comparisons are undertaken at four flux-tower sites in North America covering a range of hydroclimatic conditions and biomes. Preliminary results at the daily time step for a 4-yr period (2001–04) show good correlation (R2 = 0.89) and low bias (0.34 mm day−1) for three of the more humid sites. Results are further improved when aggregated to the monthly time scale (R2 = 0.95, bias = 0.31 mm day−1). Performance at the semiarid site is less satisfactory (R2 = 0.95, bias = 2.05 mm day−1 at the daily time step). In general, the MODIS-based daily PET estimates derived in this study are promising and show the potential for use in theoretical and operational water resource studies in both gauged and ungauged basins.

New findings about the complementary relationship-based evaporation estimation methods

Summary A novel approach has been found to estimate the equilibrium surface temperature (Te) of wet environment evaporation (Ew) on a daily basis. Employing this temperature in the Priestley–Taylor equation as well as in the calculation of the slope of the saturation vapor pressure curve with pan measurements improved the accuracy of long-term mean evaporation (E) estimation of the Advection–Aridity (AA) model when validated by Morton’s approach. Complementarity of the potential evaporation (Ep) and E terms was considered both on a daily and a monthly basis with the involved terms always calculated daily from 30 yr of hourly meteorological measurements of the 1961–1990 period at 210 SAMSON stations across the contiguous US. The followings were found: (a) only the original Rome wind function of Penman yields a truly symmetric Complementary Relationship between E and Ep which makes the so-obtained Ep estimates true potential evaporation values; (b) the symmetric version of the modified AA model requires no additional parameters to be optimized; (c) for a long-term mean value of evaporation the modified AA model becomes on a par with Morton’s approach not only in practical applicability but also in its improved accuracy, especially in arid environments with possible strong convection; (d) the latter two models yielded long-term mean annual evaporation estimates with an R2 of 0.95 for the 210 stations, which is all the more remarkable since they employ very different approaches for their Ep calculations; (e) with identical apparent Ep values the two models yielded practically identical long-term mean annual evaporation rates; (f) with the proper choice of the wind function to estimate apparent Ep the long-term mean annual E estimates of the modified AA model are still very close (R2 = 0.93) to those of the Morton approach.

증발산 산정 방법들의 비교 - 잔디기준작물을 중심으로

본 연구에서는 과거 제안된 5가지의 기준증발산식으로부터 산정된 증발산량과 pan 증발량을 이용하여 상관분석을 실시하였고 pan 계수를 산정하였다. 또한 우리나라 21개 기상관측지점에서 과거 제안된 5가지의 기준증발산식들을 비교하고 그 유사성을 알아보았다. 비교 검토된 기준증발산식은 4가지 방법으로 분류하였으며, 분류된 방법 중에서 각기 대표적인 기준증발산량 산정식을 선정하여 적용하였다. 적용된 기준증발산식은 에너지와 공기동력항의 조합법에 근거한 Penman 식, 단일근원법에 근거한 FAO Penman-Monteith(FAO P-M) 식, 복사자료를 이용한 방법인 Makkink 식과 Priestley-Taylor 식, 그리고 기온자료에 근거한 방법인 Hargreaves 식 등이다. 연구지역 선정을 위하여 기상관측지점이 있는 지역의 지리 및 지형조건을 고려하였다. 사용된 기상자료는 1970년부터 5년 간격으로 8개년의 일별 기상자료를 사용하였다. 적용결과는 수치 및 시계열 도시방법을 통하여 비교하였다. 분석결과에 의하면 대부분의 지역에서 기준증발산식과 pan 증발량과는 0.9 이상의 높은 상관관계를 보이고 있으나, pan 증발량과 비교하여 회귀식의 경사가 1.0보다 크거나 작은 경향을 보이고 있다. 전국 21개 연구지역 중에서 12개 지역에서 대기온도자료에 기초한 Hargreaves 식이 FAO P-M 식과 가장 유사한 것으로 나타났는데, 이들 지역은 대구지역을 제외하고 해안지역에 위치하고 있다. 반면에 내륙에 위치한 8개 지역에서 복사량자료에 기초한 Priestley-Taylor 식이 FAO P-M 식과 유사한 것으로 나타났다. Five representative reference evapotranspiration(RET) equations were selected, and these equations were compared with pan evaporation by correlation analysis. Pan coefficients were also estimated. Furthermore, five selected RET equations were compared to find the similarity among those at the 21 meteorological stations located in South Korea. Five RET equations selected from 4 different category were Penman(combination approach), FAO Penman-Monteith(FAO P-M) (single source approach), Makkink and Priestley-Taylor (radiation approach) and Hargreaves(temperature approach) equations. In this study, the geographical and topographical conditions were considered for the selection of study stations. The daily meteorological data measured from 1970 at an interval of 5 years were applied in this study. The evapotranspiration estimates obtained by applying evapotranspiration equations were evaluated with numerical and graphical methods. The correlation coefficients between pan evaporation and RET in study stations were above 0.9 indicating very high correlation; however, the slopes of the individual regression lines show the values greater or less than 1.0. Hargreaves equation(temperature approach) shows the most similar evapotranspiration estimates to those of FAO P-M equation from 12 study stations, which are located near to seashore except Daegu station. On the other hand, Priestley-Taylor equation(radiation approach) shows the most similar evapotranspiration estimates to those of FAO P-M equation from 8 study stations, which are located in inland.

Sensitivity of wetland saturated hydraulic heads and water budgets to evapotranspiration

The sensitivity of wetland saturated hydraulic heads and water budgets to evapotranspiration (ET) was examined using a simplified hydrologic model and eight representations of ET. Estimates of ET that created the most reliable wetland saturated hydraulic heads and water budgets employed vegetation coefficients to correct potential ET, calculated by the Priestley-Taylor equation, to actual ET. The accuracy of simulated hydraulic heads generally improved by , 1 cm, however, when using the most reliable ET estimates based on vegetation coefficients. An ET estimate that used a regression-defined extinction depth created substantial errors in simulated water budgets. Specifically, the extinction-depth ET overestimated the annual actual ET by about 40% (400 mm). An ET approximation that overestimates actual ET by 400 mm annually applied in a regional hydrologic model over the 5400 km 2 area of Everglades National Park would underestimate the annual volume of water available for ground-water recharge and surface- water runoff to coastal estuaries by 2.3 billion m 3 . For comparison, this underestimation is about two thirds of the mean volume of water in Lake Okeechobee (3.8 billion m 3 ), the largest lake in Florida, and clearly demonstrates unbiased estimates of ET are necessary for reliably simulating wetland water budgets.

An Evaluation of Evapotranspiration Model Complexity Against Performance in Comparison with Bowen Ratio Energy Balance Measurements

We investigated the relevance of model complexity in 11 evapotranspiration (ET) models, both for two-step and direct predictions for sum-of-hourly, daily, and seasonal time scales and compared their performances against the Bowen ratio energy balance system-measured ET for corn (Zea mays L.) in south central Nebraska. The simplest model we used was the water equivalent of net radiation, the intermediate was the Priestley-Taylor equation, and the complex models were the ASCE-Penman-Monteith (ASCE-PM) based two-step applications (i.e., grass and alfalfa reference surfaces). We also estimated ET with the soil water balance equation from the soil water content measured using a neutron probe soil moisture meter. By constructing the models from the simple towards more complicated, the effects of different components of the energy balance on ET during the growing season were investigated. We reported coefficients of determination (R2) and Nash-Sutcliffe efficiency (R2NS), and root mean square error (RMSE) to assess the models' performance against measured data. The ET from the soil water balance approach agreed well with those measured with the BREBS throughout the growing season. Over a seasonal time scale, the simplistic radiation-based parameterizations gave similar results as those more complex models. In the early season before canopy closure is attained (stage I) and in the late season after crop physiological maturity (stage III), the model predictions significantly deviated from measured data for both daily and seasonal estimates. The two-step ET calculation procedure was found to work very well during the mid-season (stage II) when the crop canopy was fully developed. In stages I and III, all the methods we used revealed negative R2 and R2NS values between the daily predicted and measured ET. We found significant sensible heat losses, indicating low rates of transpiration in both stages I and III. Compared to stage II, the time-dependent crop coefficients were found less capable in scaling the reference ET to represent daily actual crop ET fluctuations in stages I and III. Both the ASCE-PM-based two-step estimates as well as the direct estimates from the Priestley-Taylor and other methods gave more accurate results under full canopy cover. In this period, a sensible heat parameterization implicit in the models improved the daily predictions approximately 10% to 15%, despite model deficiencies. In the mid-season, our results suggested that the daily ASCE-PM reference grass-based approach provided arguably better estimates with respect to both daily and seasonal total fluxes compared to the sum-of-hourly calculations. Compared to the water equivalent of net radiation method, the complex models achieved up to 17% improvement in explaining the variability in daily ET during the mid-season. However, these methods did not improve seasonal ET estimates. Our results suggested that radiation is the dominant driver of evaporative losses over seasonal time scales. Other meteorological variables gained importance at daily and sum-of-hourly calculations.